The invention relates, in general, to refining molten aluminum and, more particularly, to a method for removing alkali metal impurities from molten aluminum with a halogen-containing gas while substantially avoiding the emission of corrosive or environmentally harmful gases and fumes.
Akali metals and calcium are, for the most part, harmful impurities in aliminum alloys having commercial use. Sodium is especially harmful if the hydrogen content of the alloy has not been sufficiently reduced by degassing, or the finished metal product contains magnesium. Thus, it is generally considered necessary that the sodium content in an aluminum alloy containing more than about 1% magnesium be kept below 0.0005% sodium if fabrication defects, such as edge cracking, are to be avoided during hot rolling. The presence of lithium in aluminum is also undesirable because it enhances the corrosion of aluminum foils by moist air.
Alkali metals enter aluminum in the reduction cells during electrolytic reduction of alumina in the presence of cryolite. The resulting primary aluminum is thereafter generally transferred to the cast house, where the desired alloying constituents such as magnesium are added to the melt in a mixing furnace. The alloyed metal is subsequently fluxed with chlorine to reduce its concentration of hydrogen and alkali metal impurities. The treated molten alloy is then cast into ingots.
The traditional methods of removing alkali metals from aluminum on a commercial scale fall into one of three categories: (1) holding the metal in the molten state for an extended period of time; (2) solidifying the metal by casting it into ingots and remelting same; and (3) chlorine fluxing the molten metal with chlorine, chlorine-nitrogen mixtures or with halide-containing salts at the casting station after the addition of the desired alloying constituents. The reduction of the alkali metal content of aluminum during holding or remelting operations is attributable to the high affinity of these impurities toward oxygen, and to the fact that the solubility of sodium in solid aluminum is extremely low. During the conventional chlorine fluxing of aluminum alloys, the sodium content of the metal is reduced by the chemical reaction between sodium and chlorine.
Although all of the above-described methods are successful to varying degrees in removing sodium from aluminum melts, they nevertheless, have serious disadvantages insofar as the cost and efficiency of the overall refining operation is concerned. Holding the metal in the molten state, for example, is both time consuming as well as ineffective in reducing the alkali content of the melt to the very low levels desired in the finished product, since the partial pressure of these impurities over molten aluminum is very low. Casting the metal into ingots and remelting the metal is similarly inefficient with respect to time and energy demands. Consequently, primary aluminum is normally transferred from the reduction plant to the cast house in the molten state rather than in the solid state.
Chlorine fluxing of the molten alloyed metal at the casting station has several serious drawbacks. One serious disadvantage is associated with the emission of corrosive and toxic gases and fumes. This undesirable emission consists mainly of unreacted chlorine, aluminum chloride, hydrochloric acid mist and aluminum oxide fume, the latter two compounds being produced by hydrolysis from the aluminum chloride gas. Consequently, the general use of chlorine for removing hydrogen and other impurities from aluminum alloys is being restricted by pollution control legislation, with the result that greater emphasis is being placed on non-polluting refining methods such as, sparging the metal with an inert gas, or by molten metal filtration techniques such as described in U.S. Pat. Nos. 3,737,303 and 3,373,304.
The second disadvantage of removing sodium with chlorine or with halide-containing salts relates to the fact that in magnesium-containing alloys the reaction of chlorine with magnesium is favored relative to that with sodium, particularly at low sodium concentrations. The high affinity of chlorine toward magnesium causes serious difficulties in producing acceptable quality metal, especially in high magnesium alloys where a low sodium level is specifically desired to avoid edge cracking. Thus, large amounts of chlorine must be reacted with these alloys, far in excess of that required for the stoichiometric removal of sodium, in order to reach the very low levels of sodium which are desired. This excess chlorine is not only wasted but is directly responsible for the emission of pollutants and for the loss of magnesium from the alloy. The costly loss of magnesium is inherent in all of the presently known sodium removal processes irrespective of whether the fluxing of the alloy is performed with halogen gases or with halide-containing salts. Therefore, it should be apparent that a refining operation which can readily remove alkali metals from aluminum with no appreciable metal loss, which avoids the wasteful use of chlorine and does not produce prohibitive amounts of atmospheric pollutants has numerous advantages over the present state of the art.